Instability-Induced Breakdown in Dual-Frequency Capacitively Coupled Plasmas

Capacitively coupled plasmas (CCPs) are commonly used in semiconductor fabrication as they allow precise control of ions and neutral radicals over large substrates. For high-aspect ratio etching and deposition, low-pressure operations are promising because more accelerated ions can be directed normal to the substrates. However, as the gas pressure decreases, the minimum threshold for plasma formation is reached (also known as breakdown). This phenomenon is well understood for dc discharges (as described by Paschen curves), but less understood for rf discharges. A 1D particle-in-cell, Monte Carlo collision coupled model is developed to investigate kinetic and non-Maxwellian behavior of a dual frequency, low temperature CCP at low pressures. A parametric study is performed varying the gas pressure from 2.6 Pa to 0.09 Pa at room temperature, below which the plasma is not formed. Preliminary plasma density and velocity distribution results suggest that at low pressures, bump-on-tail (or two-stream) instabilities are observed, which may play an important role in the dual-frequency rf breakdown.